Introduction The objective of the study is to better understand the root front of two Poaceae and a leguminous known to be drought resistant. The results will be compared considering that in a deep but poor soil, the progression of the root front (RF) is a key parameter to understand the water and mineral uptake of these crops. Methods Root biomass and root length density (RLD) were measured in a deep sandy soil of Senegal using the monolith method, the soil was trenched by 10 cm layers from the soil surface to the root front, on a 1m2 of soil surface for millet and sorghum and 0.25 m2 for groundnut. Measurements were made at several dates between one month after planting (DAP) and harvesting to allow the study of the RF dynamic. For millet and sorghum studies 67 and 58 monoliths of 1m2 were measured. For groundnut, 61 monoliths of 0.27m2 were measured. Roots were extracted from soil by sieving 2 mm mesh and weighed. The root length density was measured by the Newman - Tennant method. Results and Discussion Millet had the fastest root front growth: 3.5 cm/day between emergence and flowering (60 DAP). RF progression was a little slower after flowering to reach a depth of 2 meters at harvest (90 DAP). The progression of root front of groundnut was constant (2.7 cm/day) between emergence and the end of flowering (55 DAP) and then reached 1.5 meter. The root front did not increase between 55 DAP and 95 DAP (harvesting). The values of the RF for millet and groundnut were close at the end of crop cycle but with a different dynamic. The variety of tropical sorghum studied had a short crop cycle (110 days).The progression of RF was only 1cm/day, less than pearl millet. Therefore, the depth of the root system was only 1.1 m at harvest. However, the biomass of the root system for sorghum (90 g/m2 at harvest) was higher than millet (38 g/m2). The root system of sorghum remained mainly in the surface layers of soil. Conclusion The monoliths method is hard to use and time consum